This invention relates to the determination of the composition of electrically conductive mixtures, and, more particularly, to the determination of the composition of a gasoline-alcohol mixture using a compensated dielectric measurement.
It is sometimes necessary to determine the composition of a mixture which is electrically conductive because one or more of the components of the mixture are electrically conductive. One of the most important of such situations is the determination of the composition of a gasoline-alcohol mixture used as the fuel in the internal combustion engine of an automotive vehicle. In some areas, alcohol is added to gasoline for economic and environmental reasons. It is necessary to vary the control settings of the engine that uses such a fuel responsive to the alcohol content and composition of the mixture, in order to ensure its clean, efficient operation.
To control the engine settings responsive to the composition of the fuel, it is first necessary to measure the composition of the fuel in a reliable fashion. Techniques and apparatus for making such measurements of automotive fuels in real time are known. U.S. Pat. Nos. 5,231,258, 5,103,184, and 5,089,703 disclose a fuel sensor that makes such measurements capacitively. This approach is operable and reasonably accurate in many situations.
However, a potential inaccuracy in the composition measurements arises because the fuel may contain impurities such as salts that cause variations in its electrical conductivity even at constant composition and temperature. The dielectric properties of the fuel, upon which the capacitance measurements are based, vary somewhat according to its electrical conductivity. In particular, the actual composition of a highly conductive gasoline-alcohol fuel mixture may be somewhat different from that reported from readings of the capacitive sensor, because of such electrical conductivity effects.
In one approach to overcoming this problem, two sensors are provided, one to measure the capacitance of the fuel and the other to measure the electrical conductivity of the fuel. This approach is expensive in that two separate sensors are required, and may lead to instrumentation-based inaccuracies due to changes in characteristics of the two sensors over time.
There is a need for an improved approach to measuring the composition of a mixture such as a gasoline-alcohol mixture, which is both inexpensive to perform and accurate. The present invention fulfills this need, and further provides related advantages.